1. Field of the Invention
The present invention relates to a method of making an electronic lapping guide (ELG) for lapping a read sensor and, more particularly, to an ELG which is composed of a different material than the read sensor so that the ELG is not constrained by the resistance of the sensor nor its magnetoresistance (MR).
2. Description of the Related Art
A typical magnetoresistive (MR) read head includes an MR read sensor which is located between first and second nonmagnetic insulative read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. First and second lead layers are connected to the read sensor for conducting a sense current therethrough. When the read sensor is subjected to positive and negative signal fields from tracks on a rotating magnetic disk in a disk drive the resistance of the read sensor changes. These resistance changes cause potential changes in the sense current circuit, which are processed as playback signals by processing circuitry. The read head has an exterior head surface which faces the rotating magnetic disk and is supported on swirling air from the rotating disk which is referred to as an air bearing. For this reason the head surface is referred to as an air bearing surface (ABS). The read sensor has a back edge which is recessed in the read head opposite the air bearing surface. The back edge is precisely located by photolithography processing. During construction the ABS must also be precisely defined so that the read sensor has a precise stripe height which is the distance between the ABS and the back edge. This is accomplished by lapping (grinding) a wafer on which the MR head is constructed until the ABS is achieved.
The stripe height of MR sensor is determined by lapping the head structure while measuring the resistance of an electrical element. This electrical element is called an electronic lapping guide (ELG). The ELG and the sensor are formed from the same material and the back edge of this ELG is formed in the same photo and subtractive processes as the back edge of the sensor element, i.e. the ELG back edge and the sensor back edge are self-referenced to each other. To the first order the resistance of the ELG, R is inversely proportional to the height of the sensor, i.e.
RELG=xcfx81xc3x97lxc3x971/txc3x971/hxe2x80x83xe2x80x83(1)
where h is the height of the ELG, l is the length of the ELG, t is the thickness of the ELG, and xcfx81 is the resistivity of the ELG layers. The quantity xcfx81/t is called the sheet resistance Rs. So, the ELG resistance is then
RELG=Rsxc3x97lxc3x971/hxe2x80x83xe2x80x83(2)
One problem with the present ELG strategy occurs when smaller sensor heights are required. For present day spin valve heads, Rsxcx9c15 ohms/square, l=10 xcexcm, and the target stripe height is 0.5 xcexcm. For proper lapping to the target stripe height, a final ELG resistance of 300 ohms is required. Future spin valve heads may have Rsxcx9c20 ohms/sq but with target stripe heights as low as 0.1 xcexcm to 0.2 xcexcm which results in final ELG resistances up to a factor of 5 greater than present spin valve heads. This increase in final resistance will invariably require modification of lapping algorithms and electronics to sense the higher resistance. Although the geometry of the lapping guide can be changed to reduce the final resistance, i.e. decrease the ELG length or offset the back edge of the ELG relative to the sensor element, the lapping precision is degraded since the rate of resistance change versus ELG height is also reduced by these changes.
A second problem with the present ELG strategy occurs when new sensor materials with higher magnetoresistance, DR/R characteristics are used. Future spin valves will have DR/R in the 15% range. When the ELG is formed using this sensor material, the Rs of the ELG can vary by 15% depending on the orientation of the magnetization of the free and pinned layers. These orientations can be perturbed by external fields or by stress induced by the lapping process. Such a Rs change can effect final stripe height by 15% independent of the lapping algorithm precision.
A third problem with the present ELG strategy occurs when xe2x80x9ccurrent perpendicular to the planexe2x80x9d (CPP) structures are used, such as tunnel valve sensors. The sheet resistance of such structures as measured across the planes is typically low,  less than 0.1 ohm/sq, making the resistance of the ELG too low for accurate measurements of resistance changes during lapping. This reduces final ELG resistances by a factor of 100 or to values on the order of 2 to 4 ohms. This is particularly so for tunnel valve structures where the sensor consists of capping layers composed of noble metals (Pt, Pd, Rh, Au, Cu).
The aforementioned problems are overcome by constructing the ELG from a different material than the read sensor with predetermined electrical resistivity and magnetoresistance. After the sensor material is deposited, a photolithography and liftoff process is used to locally remove sensor material and deposit ELG material. The ELG material layer has the same milling endpoint time as the sensor material layer. When this condition is satisfied the process step, which forms the back edge of the ELG and the sensor (i.e. photolithography and ion milling to the endpoint), will create self-aligned and self-referenced back edges for both the sensor and the ELG.
For tunnel junction sensors the leads are designed so that the current flows perpendicular to the planes of the major thin film surfaces of the sensor (CPP) while in a spin valve sensor the leads are designed so that the current flows parallel to the planes of the major thin film surfaces of the sensor (CIP). In both instances the leads of the ELG are designed so that a current flows parallel to the major thin film surfaces of the ELG. In the present invention two separate lead structures are formed by separate photolithography steps but the back edge of both the ELG and the spin valve sensor are still formed in a single process step.
An object of the present invention is to provide an ELG and a method of making wherein the ELG is constructed from a different material than a read sensor so that the ELG is not constrained by the resistance of the sensor nor its magnetoresistance.
Another object is to provide and make an ELG for lapping read sensors wherein the ELG does not have to be changed when the material of the read sensor is changed.